The present invention relates to improved gas distribution assemblies for use in fuel cells, fuel cells employing such elements, and processes and apparatus for making such elements.
Reference is hereby made to other related patent applications which are assigned to the same assignee as the present application; application of H. Feigenbaum and A. Kaufman entitled "Integral Gas Seal For Fuel Cell Gas Distribution Plate", Ser. No. 430,453, Filed on 9/30/82; application of H. Feigenbaum and S. Pudick entitled "A Process For Forming Integral Edge Seals In Porous Gas Distribution Plates Utilizing A Vibratory Means", Ser. No. 430,291, Field on 9/30/82 and application of H. Feigenbaum, S. Pudick and R. Singh entitled "Edge Seal For Porous Gas Distribution Plate Of A Fuel Cell," Ser. No. 430,142, Filed on 9/30/82 and now U.S. Pat. No. 4,459,212.
Fuel cell design and operation generally involves conversion of a hydrogen-containing fuel and some oxidant via an exothermic reaction into D.C. electrical power. This reaction is well-known and has established parameters and limitations. It has been known for some time that fuel cells can be extremely advantageous as power supplies, particularly for certain applications such as a primary source of power in remote areas. It is highly desirable that any such cell assembly be extremely reliable. Various fuel cell systems have been devised in the past to accomplish these purposes. Illustrative of such prior art fuel cells are those shown and described in U.S. Pat. Nos. 3,709,736, 3,453,149 and 4,175,165. A detailed analysis of fuel cell technology comparing a number of different types of fuel cells appears in the "Energy Technology Handbook" by Douglas M. Consadine, published in 1977 by McGraw Hill Book Company at pages 4-59 to 4-73.
U.S. Pat. No. 3,709,736, assigned to the assignee of the present invention, describes a fuel cell system which includes a stacked configuration comprising alternating fuel cell laminates and electrically and thermally conductive impervious cell plates. The laminates include fuel and oxygen electrodes on either side of an electrolyte comprising an immobilized acid, U.S. Pat. No. 3,453,149, assigned to the assignee of this invention, is illustrative of such an immobilized acid electrolyte. The fuel cells further comprise gas distribution plates, one in electrical contact with the anode and one in electrical contact with the cathode. The gas distribution plates conduct the reactant materials (fuel and oxidant) to the fuel cell.
In order to electrically interconnect a group of discrete cells to form one larger fuel cell stack, bipolar assemblies are commonly used. For instance, in U.S. Pat. No. 4,175,165, assigned to the assignee of the present invention, a stacked array of fuel cells is described wherein reactant gas distribution plates include a plurality of gas flow channels or grooves for the distribution of the reactants. The grooves for the hydrogen gas distribution are arranged orthogonally relative to the grooves for the oxygen distribution.
The gas distribution plates themselves, whether they are part of termination assemblies having individual distribution plates for one or the other of the reactants or bipolar assemblies having two distribution plates for distributing both reactants in accordance with this disclosure, are formed of an electrically conductive impervious material. Where bipolar plates are prepared from a non-porous material, such as aluminum, the plate is typically coated with a layer of non-corrosive material, such as gold, so as to effectively isolate it from the corrosive agents, such as the electrolyte, within the fuel cell environment. In more recent fuel cell designs, the gas distribution plates of such assemblies are formed of a porous material so that a more uniform and complete flow of gas over the electrode surface is provided.
In previous systems wherein nonporous gas distribution plates were utilized, the reactants always flowed only through the grooves and were contained by the walls thereof. However, in the more recent systems utilizing porous plates, it has been necessary to seal the porous plates along the edges, and in bipolar assemblies, to segregate the reactants from one another to avoid their unintended mixing which could cause the cells to operate improperly or fail altogether.
Various techniques for sealing such porous gas distribution plates are known. In one such approach, an impervious plate is placed between the gas distribution plates forming a bipolar assembly to prevent the reactants from mixing together. In another prior art approach, a sealed bipolar plate is made up of a porous carbon plate layer which is first grooved to provide the reactant channels. Then, five or six layers of suitable material such as a resin or carbon material are placed on or impregnated into all surfaces. However, in this arrangement, the sealing layer is very thin and if damaged, exposes the original porosity of the porous carbon plate. Although this technique precludes unintended gas transmission, it can result in inadequate electrical contact between such contiguous layers and cells.
In the area of cooling assemblies typically used in larger stacks of fuel cells, a technique has been devised in which a sealant film and additional conductive materials are sandwiched between two plates to provide a bridging electrical contact across the interfacial boundary which separates them. This technique is disclosed in a copending, commonly assigned, U.S. Application entitled "Film Bonded Fuel Cell Interface Configuration" by A. Kaufman and P. Terry, Ser. No. 430,148, Filed on 9/30/82, and now abandoned. This arrangement provides effective containment of free electrolyte, a corrosive agent, from the cooling assembly as well as good electrical conductivity. However, it is readily apparent that this approach introduces additional components at the interface of such plates which can complicate manufacture and assembly.
A number of techniques have been disclosed in the prior art relating to the preparation of plates in fuel cells. These include U.S. Pat. Nos. 2,969,315; 3,223,556; 3,479,225; 3,779,811; 3,905,832; 4,035,551; 4,038,463; 4,064,322; and 4,311,771. For instance, the U.S. Pat. No. 2,969,315 discloses a fuel cell configuration in which a bipolar plate is fabricated by deposition of two layers of porous nickel on opposite sides of a common support. This common support effectively precludes gas transmission between the two porous nickel layers. Each of the two porous layers can be formed from a nickel powder by sintering the powder on the support layer.
The U.S. Pat. No. 3,223,556 discloses a fuel cell configuration in which a gas impermeable layer, moistened with electrolyte, is disposed intermediately between two layers of porous material containing the deposited catalyst on its respective surface opposite the gas impermeable layer. Electrical contact between these porous layers is achieved through an external circuit which connects an electrical grid within each catalyst to an incandescent lamp. The gas impermeable member which separates each of the porous plates from one another does not apparently bond the two porous plates to the other. The physical integrity of this composite is maintained by some other means.
The U.S. Pat. No. 3,479,225 discloses air and oxygen depolarized electrochemical units for electrochemical generation of electric current. This device is of a modular cell construction having a replaceable modular anode. The anode illustrated for this device is itself of a composite construction whereby two separate parallel plates of the anode module are bisected by an insulating layer. This insulating layer is gas transmissive but exclusive of fuel transfer therebetween. The U.S. Pat. No. 431,771 discloses a permselective bipolar membrane for electrodialyte cells, the permselective membranes comprising two layers of weakly dissociated ion exchange materials in intimate contact with one another.
The U.S. Pat. Nos. 3,779,811, 4,064,332, and 4,038,463 disclose a system for maintaining the proper fluid balance within a fuel cell by separation of the volume tolerance of the cell from its electrochemical balance. This is achieved by providing a porous back-up plate to each anode and cathode. This plate serves as a reservoir for storing excess fluid produced during the electrochemical reaction of oxygen and hydrogen and for replenishment of electrolyte which is lost as a result of high temperature operation. Each of the porous back-up plates is connected to either the anode or the cathode by means of a series of porous pins. The U.S. Pat. No. 4,064,332 patent discloses that the catalyst containing layer contiguous to the electrolyte reservoir is impregnated with a hydrophobic material "to a shallow depth". This hydrophobic material is impermeable to electrolyte yet permeable to gas, thereby permitting gas accessability to the catalyst.
In the construction of bipolar assemblies, as well as other assemblies used in fuel cells such as current collecting assemblies and cooling assemblies wherein distribution of the reactants takes place, it is apparent that the effective containment of reactant materials is important. In such assemblies it is also equally apparent that the assemblies should maintain electrical continuity and, in some cases, also provide a barrier against corrosive agents which are a necessary part of the stack from reaching those regions within the stack that would be adversely affected thereby. Accordingly, the invention disclosed herein provides an integral gas seal for gas distribution assemblies for use in fuel cells. It also provides process and apparatus for making such an assembly.